1. Field of the Invention
The present invention relates to a method of making chalcogenide glasses. More particularly, the invention relates to a method of making selenium based chalcogenide glasses utilizing liquid encapsulation.
2. Description of the Related Art
Chalcogenide glasses consists of one or more of the elements sulfur (S), selenium (Se), or tellurium (Te). Chalcohalides are glasses containing one or more of the elements S, Se and Te, and one or more halides anions (F, Cl, Br and I). Chalco-oxides are glasses containing one or more of the elements S, Se and Te and oxygen. Chalcogenide glasses are of interest because of their ability to transmit infrared radiation. Selenium based chalcogenide glasses (for example, As2Se3 and Ge28Sb2Se60) are of particular interest due to their chemical durability, moderate glass transition temperatures, and ability to transmit infrared radiation both in the 3-5 xcexcm and 8-12 xcexcm regions.
Chalcogenide glasses have been made by a number of techniques. U.S. Pat. Nos. 3,338,728, 3,343,972, and 3,360,649 disclose the production of chalcogenide glasses in sealed ampoules similar to that discussed below in relation to the commercial chalcogenide glass known as TI-1 173. U.S. Pat. No. 3,360,649 relates specifically to a selenium-germanium-antimony chalcogenide glass.
U.S. Pat. No. 4,484,945 discloses a process which involves subjecting a mixture of chalcogenide oxides contained in solution to a simultaneous coreduction reaction. The coreduction reaction is achieved by adding reducing agents to the oxide solution such as hydrazine, sulphur dioxide, thioureas, etc.
U.S. Pat. No. 4,492,763 discloses germanium-free chalcogenide glasses prepared utilizing sealed ampoules.
High purity oxide glasses may be prepared by melting the oxides together in an open crucible. For selenium based chalcogenide glasses, high purity metal selenides are generally not commercially available, therefore, selenium metal is melted and reacted to form chalcogenide glass. Selenium melts at 216xc2x0 C. and its vapor pressure reaches I atmosphere at 685xc2x0 C. while the melting points of most other chalcogenide glass components are greater than 700xc2x0 C. Therefore, reaction in an open crucible will result in the almost complete evaporation loss of selenium.
One selenium based chalcogenide glass was formerly made by Texas Instruments, now Raytheon Company, and is known as TI-1 173. TI-1 173 is a ternary glass composition made according to the formula Ge28Sb12Se60. To prevent the evaporation loss of selenium during the making of TI-1173, the reaction is conducted in a sealed quartz ampoule. The starting materials including selenium metal are placed in a quartz tube and the tube is sealed while under a vacuum. The tube is slowly heated and mixing of the molten metals occurs by rocking and/or rolling the tube during heating. The tube is then quenched to form the chalcogenide glass. The quartz tube is sacrificed to remove the reacted glass. While this method allows for commercial production of TI-1173, there are a number of shortcomings with this method, including: (1) during sealing of the quartz tube, oxygen, an impurity, may be introduced, degrading IR transmission; (2) there is a risk of explosion of the sealed quartz tube during heating if the ampoule is not designed and/or heated properly to prevent sublimation of the selenium melt; (3) the rock and/or roll mixing is not sufficient to produce optical quality glass; the reacted glass must be re-melted, stirred, re-cast and annealed to produce optical quality glass; (4) the quartz ampoule is not reusable, (5) temperatures in excess of 900xc2x0 C. must be reached to completely melt the germanium; and (6) glass batch size is limited by the size of commercially available quartz tubing, and by the margin of safety required to reduce the risk of explosion.
Accordingly, a need has arisen for making chalcogenide glass, and particularly selenium based chalcogenide glass, in a safer and more economical fashion.
In accordance with the present invention, a method of producing chalcogenide glass is provided that significantly improves the safety and economy of making the glass. The method includes the steps of: (1) placing about stoichiometric amounts of glass components into a reactor, (2) substantially covering the glass components in the reactor with an encapsulent to prevent the evaporation loss of low boiling point or high vapor pressure glass components, and (3) heating the glass components to a temperature below the boiling points of the components, and (4) actively mixing the components to cause the liquid glass components to react with the solid glass components to form molten chalcogenide glass.
Accordingly, an object of the present invention is to provide an improved method for producing selenium based chalcogenide glasses which is safer and more economical than known methods.